From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f197.google.com (mail-pf0-f197.google.com [209.85.192.197]) by kanga.kvack.org (Postfix) with ESMTP id A9C6B6B000C for ; Fri, 13 Apr 2018 08:14:36 -0400 (EDT) Received: by mail-pf0-f197.google.com with SMTP id f19so4427446pfn.6 for ; Fri, 13 Apr 2018 05:14:36 -0700 (PDT) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id t16si4536380pfj.10.2018.04.13.05.14.35 for (version=TLS1 cipher=AES128-SHA bits=128/128); Fri, 13 Apr 2018 05:14:35 -0700 (PDT) Date: Fri, 13 Apr 2018 14:14:33 +0200 From: Michal Hocko Subject: Re: [PATCH] memcg: Remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure Message-ID: <20180413121433.GM17484@dhcp22.suse.cz> References: <20180413085553.GF17484@dhcp22.suse.cz> <20180413110200.GG17484@dhcp22.suse.cz> <06931a83-91d2-3dcf-31cf-0b98d82e957f@virtuozzo.com> <20180413112036.GH17484@dhcp22.suse.cz> <6dbc33bb-f3d5-1a46-b454-13c6f5865fcd@virtuozzo.com> <20180413113855.GI17484@dhcp22.suse.cz> <8a81c801-35c8-767d-54b0-df9f1ca0abc0@virtuozzo.com> <20180413115454.GL17484@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Kirill Tkhai Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, vdavydov.dev@gmail.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On Fri 13-04-18 15:07:14, Kirill Tkhai wrote: > On 13.04.2018 14:54, Michal Hocko wrote: > > On Fri 13-04-18 14:49:32, Kirill Tkhai wrote: > >> On 13.04.2018 14:38, Michal Hocko wrote: > >>> On Fri 13-04-18 14:29:11, Kirill Tkhai wrote: > > [...] > >>>> mem_cgroup_id_put_many() unpins css, but this may be not the last reference to the css. > >>>> Thus, we release ID earlier, then all references to css are freed. > >>> > >>> Right and so what. If we have released the idr then we are not going to > >>> do that again in css_free. That is why we have that memcg->id.id > 0 > >>> check before idr_remove and memcg->id.id = 0 for the last memcg ref. > >>> count. So again, why cannot we do the clean up in mem_cgroup_free and > >>> have a less confusing code? Or am I just not getting your point and > >>> being dense here? > >> > >> We can, but mem_cgroup_free() called from mem_cgroup_css_alloc() is unlikely case. > >> The likely case is mem_cgroup_free() is called from mem_cgroup_css_free(), where > >> this idr manipulations will be a noop. Noop in likely case looks more confusing > >> for me. > > > > Well, I would really prefer to have _free being symmetric to _alloc so > > that you can rely that the full state is gone after _free is called. > > This confused the hell out of me. Because I _did_ expect that > > mem_cgroup_free would do that and so I was looking at completely > > different place. > > > >> Less confusing will be to move > >> > >> memcg->id.id = idr_alloc(&mem_cgroup_idr, NULL, > >> 1, MEM_CGROUP_ID_MAX, > >> GFP_KERNEL); > >> > >> into mem_cgroup_css_alloc(). How are you think about this? > > > > I would have to double check. Maybe it can be done on top. But for the > > actual fix and a stable backport potentially should be as clear as > > possible. Your original patch would be just fine but if I would prefer > > mem_cgroup_free for the symmetry. > > We definitely can move id allocation to mem_cgroup_css_alloc(), but this > is really not for an easy fix, which will be backported to stable. > > Moving idr destroy to mem_cgroup_free() hides IDR trick. My IMHO it's less > readable for a reader. > > The main problem is allocation asymmetric, and we shouldn't handle it on free path... Well, this is probably a matter of taste. I will not argue. I will not object if Johannes is OK with your patch. But the whole thing confused hell out of me so I would rather un-clutter it... -- Michal Hocko SUSE Labs